#811 (reopen of #792, carlosjfcasero, Emby/Jellyfin): "append" mode clobbered
the playlist's custom image. The post-sync image push only excluded
'reconcile' — so append (which edits the playlist in place via
append_to_playlist) still re-pushed the source image over the user's poster
every sync. Now both in-place modes (reconcile + append) skip the image push;
only the destructive 'replace' (recreate-from-scratch) pushes it.
append_to_playlist + set_playlist_image were verified to NOT touch tracks or
description (image push only POSTs /Images/Primary), so this is the identity-
clobber fix for append.
Tests: append + reconcile preserve the image, replace still pushes it.
40e3dac881